Re: Recycled: Measuring Coupling Coefficients

From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Wednesday, December 03, 1997 4:19 PM
To: 	Tesla List
Subject: 	Re: Recycled: Measuring Coupling Coefficients

Guess I have something to say here:

> From:   John H. Couture[SMTP:couturejh-at-worldnet.att-dot-net]
> Sent:   Monday, December 01, 1997 11:27 PM
> To:     Tesla List
> Subject:    Re: Recycled: Measuring Coupling Coefficients

>   I agree that the K factor (coupling coefficient) appears to be independent
> of the frequency for Tesla coils. However, further research indicates the K
> factor is a number that depends on the type of operation. Refer to Terman's
> Radio Engineers' Handbook. For example, in radio circuits the K factor
> varies with the frequency because of the variable  tuning capacitor.
> Changing this capacitor changes the operating frequency and changes the
> coupling. This means that a Tesla coil with a fixed capacitor would have a
> fixed coupling  and be independent of frequency . However, if the TC
> capacitor is changed and the operating frequency changed the coupling would
> also be changed without changing the physical characteristics or geometry of
> the coil. Has anyone ever checked this?

My experiments suggest k is purely related to coil geometry and 
proximity given that Ls and Lp are *fixed*, not "variables". If you 
alter geometry of either coil or coil proximities to one another, Lp 
and Ls don't change but k does and hence Lm must. It is worth noting 
that unless you know how many turns you will be using on that 15 turn 
primary, you can't pre-calculate k either.


>   The mutual inductance test for K factor introduces two additional
> variables, Lp and Ls, to the voltage and current that is necessary to
> determine the Lm. This test would be less accurate compared to the other
> tests using only two variables. If all variables are  measured to the same
> accuracy the other tests would have twice the accuracy of the mutual
> inductance test.
>   As for the light bulb shown in the Tesla Coil Design Manual for the mutual
> inductance test, the non linearity of the bulb is not involved. This test
> uses a fixed current that is independent of the non linearity of the bulb.
> The fixed current is obtained by varying the primary voltage with a variac.
> This has the advantage that the mv in the secondary is the uh of the mutual
> iinductance so calculations are not necessary. This test is also safer
> because it operates at low voltage.
> Comments welcomed.
>   John Couture